Ionic Compound Naming- Rules and Examples
What Ionic Compounds Actually Are
Ionic compounds form when metal atoms lose electrons and nonmetal atoms gain electrons. The resulting charged particles (ions) stick together through electrostatic attraction. That's it. No magic, just opposite charges attracting.
The metal becomes a positive ion (cation). The nonmetal becomes a negative ion (anion). You name the compound by combining the names of these two parts.
The Basic Naming Rule
Every ionic compound follows one pattern:
Cation name + Anion name
The cation is always named first. The anion always gets the -ide suffix. This never changes, no matter what.
Binary Ionic Compounds (Two Elements)
Simple Metal + Nonmetal
When you have a metal from Groups 1, 2, or 13 (aluminum), the naming is straightforward. Just combine the metal name with the nonmetal name and slap -ide on the nonmetal.
- NaCl = Sodium + Chlorine → Sodium chloride
- K₂S = Potassium + Sulfur → Potassium sulfide
- MgO = Magnesium + Oxygen → Magnesium oxide
- Al₂O₃ = Aluminum + Oxygen → Aluminum oxide
See the pattern? The formula tells you nothing about charges—the charges already balanced, and the subscripts reflect that. Don't try to "add" subscripts into the name. The name is just the two elements.
Transition Metals: The Complication
Transition metals can form multiple different ions. Iron can be Fe²⁺ or Fe³⁺. Copper can be Cu⁺ or Cu²⁺. This is where most students mess up.
When the metal has variable charge, you must indicate that charge using Roman numerals in parentheses. This is called the Stock system.
- FeCl₂ → Iron(II) chloride (Fe²⁺)
- FeCl₃ → Iron(III) chloride (Fe³⁺)
- Cu₂O → Copper(I) oxide (Cu⁺)
- CuO → Copper(II) oxide (Cu²⁺)
The Roman numeral tells you the charge on the metal ion, not the subscript. You figure out the charge from the formula.
Old Naming System (Still Shows Up)
Some textbooks use -ous and -ic suffixes instead of Roman numerals:
- Fe²⁺ = Ferrous ion
- Fe³⁺ = Ferric ion
- Cu⁺ = Cuprous ion
- Cu²⁺ = Cupric ion
You'll see this in older sources or on periodic tables from certain publishers. The Stock system (Roman numerals) is the modern standard and what you should use.
Compounds with Polyatomic Ions
Polyatomic ions are clusters of atoms with an overall charge. You don't break them apart or change their names.
Common Polyatomic Ions to Memorize
- NO₃⁻ = Nitrate
- SO₄²⁻ = Sulfate
- CO₃²⁻ = Carbonate
- OH⁻ = Hydroxide
- NH₄⁺ = Ammonium
- PO₄³⁻ = Phosphate
- NO₂⁻ = Nitrite
- SO₃²⁻ = Sulfite
Notice the pattern: -ate has more oxygen than -ite. Sulfate (SO₄²⁻) has one more oxygen than sulfite (SO₃²⁻). Phosphate (PO₄³⁻) has one more than phosphite (PO₃³⁻).
Examples
- NaNO₃ = Sodium nitrate
- CaSO₄ = Calcium sulfate
- KOH = Potassium hydroxide
- (NH₄)₂CO₃ = Ammonium carbonate
When you have parentheses in the formula (like ammonium carbonate), it means the polyatomic ion appears multiple times. The name doesn't change—you still just name the cation and anion.
Compounds with Groups 1, 2, and 13
Metals from these groups only form one stable ion. No Roman numerals needed.
- Group 1 metals always form +1 ions (Li⁺, Na⁺, K⁺, etc.)
- Group 2 metals always form +2 ions (Mg²⁺, Ca²⁺, etc.)
- Aluminum always forms Al³⁺
These are called "fixed charge" metals. If the metal is from one of these groups, you don't indicate charge in the name. Sodium chloride, not sodium(I) chloride. Nobody does that.
Naming Quick Reference Table
| Compound Type | Example Formula | Name | Rule |
|---|---|---|---|
| Metal + Nonmetal (fixed charge) | CaO | Calcium oxide | No charge indicated |
| Metal + Nonmetal (variable charge) | PbO₂ | Lead(IV) oxide | Roman numeral required |
| Metal + Polyatomic ion | Na₂SO₄ | Sodium sulfate | Keep polyatomic name intact |
| Two polyatomic ions | NH₄NO₃ | Ammonium nitrate | Both keep their names |
How to Name Any Ionic Compound: Step by Step
Step 1: Identify the cation (positive ion). Is it a metal? If yes, check if it's a transition metal.
Step 2: If it's a transition metal, determine its charge. Use the anion to figure this out—the total positive charge must equal the total negative charge.
Step 3: Identify the anion (negative ion). If it's a single element, add -ide. If it's a polyatomic ion, use its exact name.
Step 4: Write: Cation name + Anion name. Add Roman numeral for transition metals.
Worked Example
Name: Fe₂O₃
Step 1: Fe is iron—a transition metal with variable charge.
Step 2: O is oxygen, always O²⁻. Three oxygen atoms = 3 × (-2) = -6 total charge. Two iron atoms must give +6 total charge. 6 ÷ 2 = +3. Each iron is Fe³⁺.
Step 3: O becomes oxide.
Step 4: Iron(III) oxide
Common Mistakes That Cost You Points
- Adding subscripts to the name. CoCl₂ is cobalt(II) chloride. Not "cobalt dichloride." The subscripts are already baked into the formula.
- Forgetting Roman numerals for variable charge metals. FeCl₂ is iron(II), not iron chloride. Both exist, so you must specify.
- Changing polyatomic ion names. CaSO₄ is calcium sulfate. Not calcium sulfoxide. The names are fixed.
- Getting cation and anion order wrong. Cation always comes first. NaCl is sodium chloride, not chloride sodium.
The -ide vs -ate vs -ite Rule
For binary compounds (two elements), you always use -ide:
- NaCl = Sodium chloride
- CaS = Calcium sulfide
- Mg₃N₂ = Magnesium nitride
For compounds with polyatomic ions, you use the ion's actual name—which may end in -ate, -ite, or something else entirely:
- NaNO₃ = Sodium nitrate (not sodium trioxynitrate)
- CaCO₃ = Calcium carbonate (not calcium carboxide)
Don't try to apply the -ide rule to polyatomic ions. Just memorize the ion names.
Practice: Name These Compounds
Try naming these before checking the answers:
- MgCl₂
- Cu₂S
- Fe(NO₃)₂
- Al₂(SO₄)₃
- SnO
Answers:
- Magnesium chloride
- Copper(I) sulfide
- Iron(II) nitrate
- Aluminum sulfate
- Tin(II) oxide
For #4, aluminum is in Group 13—no Roman numeral needed. For #5, tin is a variable charge metal. SnO has O²⁻, so Sn must be Sn²⁺—that's tin(II).